1. Field of the Invention
The present invention relates to a machine tool in which plural rods are arranged between a base and a traveling plate in a parallel way. More particularly, the present invention relates to a rod of the machine tool.
2. Description of the Prior Art
A Machine tool having a parallel structure have been proposed. An example of the machine tool is shown in U.S. Pat. No. 5,715,729. As shown in FIG. 1, the machine tool 50 is mainly composed of a base 51 , a traveling plate 52, six drive mechanisms 60 and six rods 55. The base 51 is fixed to an outside fixed portion through a support column 71 shown by chain double-dashed lines. To the traveling plate 52 a tool T, e.g., a drill or an end mill, is attached. Each of the drive mechanisms 60 includes a motor 65 to move a slide table 66 through a ball screw 64. One end of the rod 55 is connected to the corresponding slide table 66 through a joint 56. The other end of the rod 55 is connected to the traveling plate 52 through a joint 57.
In the configuration described above, when the motor 65 of the six drive mechanisms 60 are respectively activated in response to command signals from an unillustrated controller, the six slide tables 66 are respectively moved to swing the six rods 55 individually. The combination of the swing movements of the six rods 55 causes that the traveling plate 52 is controlled in the six-degree-of-freedom, that is position and attitude control.
In the above configuration, each of the rods 55 has five-degree-of-freedom with two kinds of the joints 56 and 57. One has two-degree-of-freedom and the other has three-degree-of-freedom. A ball joint or a universal joint are in use for the joints 56 and 57. The reason of that is to avoid the appearance of the singular positions of overmobility in the parallel structure. An example paper showing the singular positions of overmobility is Singularity Analysis of a Parallel Robot HEXA in Proceedings of the 9th Annual Conference of the Robotics Society of Japan, 1991 (on Pages 65 and 66).
However, the traveling plate 52 does not have high positioning accuracy because of a clearance between a pair of elements constituting the ball joint or the universal joint for the joints 56 and 57. This drawback cause deterioration of positioning accuracy and rigidity of the machine tool 50 with the parallel structure. Since the ball joint and the universal joint acts with the friction between pair of elements of the ball joint or the universal joint, the friction decreases durability of the machine tool 50. In addition, as the range of movement of the ball joint or the universal joint with three-degree-of-freedom is relatively narrow because of its complicated structure, the range of movement of the machine tool 50 is decreased.